Enhance syncback process by integrating serialization of service children

- Introduced `SerializationService` to serialize child instances during syncback, improving data handling.
- Updated `performSyncback` to include serialized children data in the syncback request.
- Refactored `ServiceChunk` structure to store child count instead of serialized data, optimizing memory usage.
- Adjusted related functions and tests to accommodate the new data structure, ensuring consistency and accuracy in syncback operations.

These changes significantly enhance the efficiency and clarity of the syncback process, allowing for better management of service data.

src/web/interface.rs

@@ -27,18 +27,21 @@ pub(crate) const SERVER_VERSION: &str = env!("CARGO_PKG_VERSION");
 /// Current protocol version, which is required to match.
 pub const PROTOCOL_VERSION: u64 = 6;
 
-/// A single service's children serialized as rbxm binary data, plus the
-/// service's own properties and Ref property hints.
+/// Metadata for a single service in the live syncback request.
 ///
-/// Attributes and Tags are included in the `properties` map as
-/// `Variant::Attributes(...)` and `Variant::Tags(...)` respectively,
-/// matching the encoding used by the existing `/api/write` path.
+/// Children are NOT stored here — they live in the top-level `data` blob
+/// (a single rbxm containing all children from all services). `child_count`
+/// tells the server how many consecutive children in that blob belong to
+/// this service.
+///
+/// Attributes and Tags are included in `properties` as
+/// `Variant::Attributes(...)` / `Variant::Tags(...)`.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct ServiceChunk {
     pub class_name: String,
-    #[serde(with = "serde_bytes")]
-    pub data: Vec<u8>,
+    #[serde(default)]
+    pub child_count: u32,
     #[serde(default)]
     pub properties: HashMap<String, Variant>,
     #[serde(default)]
@@ -64,12 +67,15 @@ pub struct SyncbackRequest {
     pub protocol_version: f64,
     pub server_version: String,
     pub place_id: Option<f64>,
+    #[serde(with = "serde_bytes", default)]
+    pub data: Vec<u8>,
     pub services: Vec<ServiceChunk>,
 }
 
 /// Payload passed from the API handler to the serve loop after validation.
 #[derive(Debug)]
 pub struct SyncbackPayload {
+    pub data: Vec<u8>,
     pub services: Vec<ServiceChunk>,
 }